1. Field of the Invention
The present invention pertains generally to the severing of glass, and more particularly to the cutting, piercing or edging of so-called heat strengthened glass, that is, glass having a surface compression in the range defined by United States government standards and generally understood in the industry as being heat strengthened, by means of an abrasive fluid jet directed against the glass.
2. Description of the Prior Art
Strengthening of glass may be accomplished by heating the glass to a temperature above its strain point but below its softening point, and then rapidly chilling it as by blowing cooler air against its surfaces, whereupon the surfaces or external layers of the glass are placed in compression and the core is placed in tension. Such strengthening of the glass produces a highly desirable improvement in the mechanical properties of the glass and causes it, when severely damaged as by a heavy blow or scratching of the compressive surface layer, to break into relatively harmless fragments. This latter property, whereby the glass separates into relatively harmless fragments, is highly desirable for permitting the glass to be employed as safety glazing closures as, for example, in store fronts, sky lights and other architectural glazings.
Inasmuch as severe damage to the compressive surface layer may cause the glass to fracture in a random pattern, the use of conventional glass cutting techniques involving scoring the surface and breaking along the score line, as well as the usual drilling techniques, are precluded. For that reason it has heretofore been necessary to fabricate the glass unit to its final size and configuration, and to then strengthen the glass as a final step. As will be readily apparent, such a procedure has certain disadvantages. For example, glass doors and architectural glazings are produced in many different sizes and since it has not been possible to cut the strengthened glass required by safety codes in such installations, it is necessary for replacement glass installers to either stock a great many sizes of units, or have the units custom made to the required dimensions. As a result, there may be considerable delay as well as expense in obtaining the lights, and consequently much of the replacement market has gone to substitute materials such as plastic. Also, due to the complicated shapes and special features contemplated in glazing closures for future structures, it may not be feasible to strengthen the glazing closures for these structures after they are fabricated.
The desirability of being able to cut or otherwise fabricate so-called stressed or strengthened glass has long been recognized. To that end, a number of proposals have been made to modify the internal stresses within the glass whereby even though the glass is strengthened, it may still be cut in the conventional manner by scoring the surface and then running the cut along the score line. Thus, U.S. Pat. No. 3,107,196 suggests procedures for forming stressed or tempered glass sheets wherein there is little, if any, compression at the actual surfaces of the sheets, so that the glass can purportedly be cut by conventional scoring and flexing techniques. U.S. Pat. No. 3,150,950 discloses a method for cutting, drilling or edging tempered glass wherein previously tempered glass is heated to a temperature below its strain region and then rapidly cooled to induce temporary stresses into the glass which counteract the permanent stress, and the glass is then scored and separated while the temporary stress is present. Such methods have not proven entirely satisfactory in commercial practice, particularly for cutting irregular and curved shapes from strengthened glass units as is necessary in many instances.